The present invention relates to the general field of cellular structures or cellular bodies that are used for example in sound attenuation panels for reducing the noise produced by aviation gas turbines or combustion chambers, or in reducing the weight of structural elements (stiffeners) for assemblies of the sandwich type.
These panels are typically constituted by a multi-perforated surface panel that is permeable to the soundwaves that it is desired to attenuate, and a solid reflecting panel, with a cellular structure or cellular body, such as a honeycomb, being arranged between those two walls. In well-known manner, the panels form Helmholtz type resonators that perform attenuation over a certain frequency range of soundwaves produced in the duct.
The cellular structure may be made of a metal material, as described in Documents U.S. Pat. No. 5,912,442 and GB 2 314 526, or out of composite material, i.e. a material comprising fiber reinforcement densified by a matrix, which is lighter than a metal material.
Document U.S. Pat. No. 5,415,715 discloses making honeycomb structures out of a composite material, starting from a fiber structure that is expandable. In that document, the fiber structure may be made in particular by stacking and bonding together in staggered manner two-dimensional plies of fabric so as to form a texture. The connections between the plies are made along parallel strips arranged in staggered manner and formed by adhesive or by stitching. The stack of plies is then cut into segments, with each segment then being stretched in a direction normal to the faces of the plies so as to obtain cellular structures by deformation.
In a variant embodiment described in Document U.S. Pat. No. 5,415,715, two-dimensional plies are superposed and needled together in order to form a texture. Slot-shaped cuts are then made in a staggered configuration in the texture, e.g. by a waterjet or by laser, with dimensions and at locations that define the dimensions and the shapes of the cells. After making the cuts, the texture is stretched in the direction perpendicular to the cutting planes so as to obtain a cellular structure by deformation.
Nevertheless, although. that type of fiber structure is well adapted to making cellular structures that are plane, it is found to be very difficult to use when it is desired to make cellular structures that are curved in shape. The deformation to obtain a curved shape gives rise to zones of tension in the fiber structure. When the fiber structure is made from a stack of two-dimensional plies as described above, the plies can become separated in the vicinity of these zones of tension or they can deform in non-uniform manner so as to prevent a regular cellular structure being obtained.
Furthermore, making expandable fiber textures in the manner described above requires a large number of manual operations, which are not compatible with industrial production.
There therefore exists the need to have a solution that is reliable and inexpensive, and that makes it possible to fabricate cellular structures out of composite material and of a shape that is curved.